Tall bike build

If I was to build one - I would possibly suggest, putting the headset and forks onto the bottom bike, then welding an old front axle** to the top of the headtube**. Use the old axle to bolt on the forks of the upper bike.

that won't work.

You'd need to find an old steel stem and put it in the bottom frame and weld the forks or the axle to that.

I would recommend doing it properly, find some 35mm mild steel tube and make a super long headtube, and extend the fork steerer and use ahead to connect it all up!